1. Field of the Invention
This invention relates generally to packaging methods and apparatus, and is particularly concerned with a method and apparatus for forming a seal between layers of a carton of thermoplastic coated paperboard material and the like wherein the thermoplastic material of the layers is activated by frictional heat from a vibration welding horn and then allowed to cool and set under pressure to form a seal between the layers.
2. Description of the Prior Art
The type of container with which the present invention is particularly (but not exclusively) concerned is the type made of paperboard stock coated on both sides with a thermoplastic material such as polyethylene. The thermoplastic coating is utilized not only as moisture proofing material, but also serves as an adhesive which cooperates in sealing the seams, joints and closure elements of the container so as to make the container fluid tight when it is filled with milk or other contents, and sealed and closed. An example of this general type of container is disclosed in U.S. Pat. Nos. 3,120,089; 3,309,841 and 3,910,014.
Until recently, the conventional manner of sealing thermoplastic coated paperboard cartons was to activate the thermoplastic coating, by the application of heat, to cause it to flow at the portions to be joined and sealed, and then bring the layers of the end closures into contact with each other. When the thermoplastic material between the layers cools and sets, the layers are sealed and fused together. In addition to requiring the use of elaborate and expensive apparatus for generating the considerable amount of energy required, elaborate apparatus, methods and systems are also required to dissipate the heat from the plant in which the packaging operation is being carried out. Examples of this general type of packaging apparatus are shown in U.S. Pat. Nos. 3,002,328; 3,120,089; 3,166,994; 3,187,647; 3,239,995; and 3,309,841.
U.S. Pat. Nos. 3,905,280 of Sept. 16, 1975 and 3,910,014 of Oct. 7, 1975 disclose apparatus for sealing the end closures and side seams of thermoplastic coated paperboard containers by vibration welding wherein mechanical energy is converted into localized heat energy to minimize the amount of heat and energy required, and to eliminate the necessity for expensive systems and methods for dissipating the excess heat. U.S. Pat. No. 3,526,792 discloses an ultrasonic transducer having a converter for converting electrical energy into mechanical energy, and a concentrating horn for concentrating the mechanical energy into vibrations at a desired frequency. In the above mentioned U.S. Pat. Nos. 3,905,280 and 3,910,014, the horn vibrates a predetermined frequency when energized, and cooperates with an anvil or other back-up means to clamp the layers of the thermoplastic coated paperborad between the working surface of the horn and back-up or work surface of the anvil. When the thermoplastic coated paperboard layers are claimed between the work surfaces of the horn and anvil, vibration of the horn creates frictional heat sufficient to activate the thermoplastic coating of the layers to cause the thermoplastic material of the layers to flow together. When the vibration is terminated, the thermoplastic will cool and set to form a seal between the layers and to adhesively secure the layers together throughout the area of the seal.
There are two stages in the sealing operation of thermoplastic coated paperboard containers by vibration welding. These stages are: (1) the activation stage in which the friction of the vibrating horn generates heat sufficient to activate the thermoplastic coating, and (2) the cooling stage wherein the layers are clamped together to permit the previously activated thermoplastic coating to cool and set to form the seal.
A significant factor in determining the production rate is the amount of time required in the sealing operation. Present practice is to utilize the horn in both the activation and cooling stages. When the horn is used in both stages, a complete cycle of forming a seal between the layers of the carton involves (1) bringing the working surfaces of the anvil (or backup means) and horn into opposed relationship on opposite sides of the layers to be joined and sealed, (2) activating the thermoplastic coating of the layers between the work surface by the frictional heat of the energized horn to cause the coating of the layers to flow together, (3) permitting the previously activated thermoplastic material to cool and set while still held between the working surfaces of the anvil and deenergized horn, and (4) separating the working surfaces of the anvil and horn to remove the carton layers after the seal has been formed. The time involved per cycle to date has been in the range of approximately one-half to two seconds, or more, depending upon the thickness of the paperboard, the thickness of the thermoplastic coating, the number of layers, and the surface area to be sealed.
A significant cost factor in the apparatus is the vibrating horn utilized to activate the thermoplastic coating. It has been conventional in vibration welding operations of this type to pressure actuate the horn to start the horn vibrating. As the working surface of the horn moves into contact with the layers of the carton to be joined together, the horn starts vibrating when a predetermined pressure on the working surface of the horn is sensed by the horn actuating controls. In a high production packaging system, the available time for the sealing operation is determined primarily by the speed at which the cartons must pass through the welding cycle as determined by the speed of other machines in the total system. For example, the welding apparatus must be incorporated into a system including machines for feeding and erecting cartons from flat blank form, such as machines of the type disclosed in Allen U.S. Pat. Nos. 3,599,541 of Aug. 17, 1971 and in Kellogg U.S. Pat. No. 3,937,131 of Feb. 10, 1976 , and filling and closing apparatus such as disclosed in Braun U.S. Pat. No. 3,910,014.
A significant expense involved in packaging systems using vibration welding is the cost of each vibration welding horn, as well as the power requirements for energizing the horn. Furthermore, the horn must be mounted with respect to the anvil or back-up means for movement between extended and retracted positions to permit the cartons to move past the horn after completion of a welding cycle.
In the type of operation described above, the horn is inactive during a significant portion of the welding cycle. The greater the number of vibration welding horns required, the greater the power requirements for the packaging operation. Furthermore, extension and retraction of the horn for a significant distance, plus pressure actuation of the horn, requires a significant amount of time in a mass production operation.
When the end closures of a thermoplastic paperboard carton are closed, and sealed, it is desirable to "stake" the layers at certain locations. By "staking" is meant that indentations are pressed into the overlapping layers at strategic locations to, in effect, form localized dams to close off likely leak paths between the layers. The indentations are pressed into the carton layers by protrusions from the surfaces of clamping devices used in the closing and sealing operation. It is not possible to provide staking protrusions in the surface of either the vibration welding horn or the anvil or mandrel against which the horn presses the paperboard layers, for the reason that burning would take place during the vibration welding operation at such protrusions. Thus, although staking is desirable to reduce the occurrence of leak paths, it has not been possible to combine a staking operation with a vibration welding operation.